A variety of techniques are currently utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including, among other things, the resistivity, porosity, and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the wellbores have been drilled. More recently, wellbores have been logged while drilling, which is referred to as measurement-while-drilling (MWD) or logging-while-drilling (LWD). One advantage of MWD techniques is that the information about the rock formation is available at an earlier time when the formation is not yet damaged by an invasion of the drilling mud. Thus, MWD logging may often deliver better formation evaluation (FE) data quality. In addition, having the formation evaluation (FE) data available already during drilling may enable the use of the FE data to influence decisions related to the ongoing drilling (such as geo-steering, for example). Yet another advantage is the time saving and, hence, cost saving if a separate wireline logging run can be avoided.
For an accurate analysis of some FE measurements, for example, neutron porosity (NP) measurements and/or neutron density (ND) measurements, and the like, it is important to know the actual downhole formation evaluation (FE) tool position in a borehole during drilling. By way of example, an 8-sector azimuthal caliper with 16 radii allows the determination of the exact center of the downhole formation evaluation (FE) tool in the borehole during drilling and a magnetometer allows the determination of the exact orientation of the detector face. These two parameters allow optimization of the environmental borehole effects, such as correction for borehole size and mud.
However, conventional corrections typically assume one of two conditions. Either (1) the downhole formation evaluation (FE) tool is eccentered (the FE tool center is eccentrically located with respect to the “true” center of the borehole and the FE tool center does not coincide with the true center of the borehole), and appropriate eccentered FE tool corrections are used, or (2) the downhole formation evaluation (FE) tool is centered (the FE tool center is not eccentrically located with respect to the true center of the borehole and the FE tool center does coincide with the true center of the borehole) and appropriate centered FE tool corrections are used.
In the eccentered case, conventionally an average eccentered correction for constant rotation of the FE tool is assumed whereby the FE tool is assumed to face the formation about 50% of the time and to face into the borehole about 50% of the time. However, the conventional approaches are not able to allow the selection of the proper environmental corrections to apply generally, lacking any way to track the FE tool center and direction with respect to the borehole center. For a non-azimuthal FE tool, for example, the conventional approaches lack any way to extrapolate between (1) the eccentered and (2) the centered cases described above, even assuming constant FE tool rotation.
While it has long been known that two-way travel time of an acoustic signal through a borehole contains geometric information about the borehole, methods of efficiently obtaining that geometric information acoustically continue to need improvement. In particular, a need exists for efficient ways to obtain such geometric information about a borehole to overcome, or at least substantially ameliorate, one or more of the problems described above. U.S. patent application Ser. No. 12/051,696 of Hassan et al., discloses a method and apparatus for evaluating an earth formation. The method includes conveying a logging string into a borehole, making rotational measurements using an imaging instrument of a distance to a wall of the borehole, processing the measurements of the distance to estimate a geometry of the borehole wall and a location of the imaging instrument in the borehole. The method further includes estimating a value of a property of the earth formation using a formation evaluation sensor, the estimated geometry and the estimated location of the imaging instrument. The method may further include measuring an amplitude of a reflected acoustic signal from the wall of the borehole. The method may further include estimating a standoff of the formation evaluation sensor and estimating the value of the property of the earth formation using the estimated standoff. Estimating the geometry of the borehole may further include performing a least-squares fit to the measurements of the distance. Estimating the geometry of the borehole may further include rejecting an outlying measurement and/or defining an image point when the measurements of the distance have a limited aperture. The method may further include providing an image of the distance to the borehole wall. The method may further include providing a 3-D view of the borehole, identifying a washout and/or identifying a defect in the casing. The method may further include using the estimated geometry of the borehole to determine a compressional-wave velocity of a fluid in the borehole. The method may further include binning the measurements made with the formation evaluation sensor.
One problem not discussed in Hassan is that of improving the signal-to-noise ratio of the reflected acoustic signals. It is well-known that the borehole mud is attenuative and dispersive. As a result of this, the reflected signals may be relatively weak and fairly narrow band, resulting in poor resolution. In addition, cuttings may be present in the mud and produce spurious reflections. Hassan uses a statistical method to identify and remove these spurious reflections. It would be desirable to have a method of imaging borehole walls and producing a borehole profile that can achieve good resolution and good signal to noise over a wide range of distances. The present disclosure addresses this need.